Dual comparator trigger circuit for glitch capture

Abstract: A glitch trigger circuit for measuring glitches that appear on power line signals includes two comparators and two independently controllable voltage trigger levels to allow proper trigger on glitches of unknown polarity. A high pass filter rejects the power line signal while passing the glitch signals to provide triggering on glitches with voltage amplitudes less than the voltage amplitude of the power line signal.

Description: Background of the Invention This invention relates generally to trigger circuits and in particular to a trigger circuit utilizing two comparators for generating a trigger signal in response to a fast electrical pulse commonly known as a glitch. Measuring an electrical signal with an instrument such as a digital storage oscilloscope often requires obtaining a trigger signal to synchronize the measurement process with the event of interest. Such a trigger signal is usually generated by a separate internal trigger circuit in the instrument to initiate the instrument measurement sequence. Triggering on a periodic waveform such as a sinusoidal 60 hertz electrical power line signal is easily done with a single comparator trigger circuit which generates a trigger circuit in response to the input signal voltage passing through a selected trigger voltage reference level. Choosing an appropriate trigger point on the power line signal is a matter of selecting an appropriate trigger voltage level. Glitches are transient voltage waveforms that do,not occur periodically. Glitches can be caused by random events, such as a lightning stroke or a switch closure. Glitch signals may be in the form of positive transient voltages, negative transient voltages, or bipolar (both positive and negative) transient voltages, depending on how they were generated. A trigger circuit is often necessary to synchronize the measurement equipment to the occurrence of the glitch since the glitch duration is often very short and its time of occurrence is not predictable. Assuming the type of glitch is known, a single comparator trigger circuit programmed with the proper trigger voltage level will allow an appropriate trigger signal to be generated, synchronizing the instrument measurement sequence with the desired transient event to be measured. A common measurement task is to evaluate power line signals for noise superimposed on the power line signal by measuring the glitches as they occur. The periodic power line signal is combined with the nonperiodic glitch signal. Successfully measuring the glitch signal requires having a trigger signal responsive to the glitch signals in order to synchronize the measurement sequence. However, glitch signals that are superimposed on a sinusoidal signal present a significant problem for conventional trigger circuits when the amplitude of the glitch signal is less than that of the power line signal or the voltage polarity of the glitch is not known. The glitch signal voltage will very likely be masked in the power line signal voltage, thereby foiling a conventional trigger circuit which is responsive only to the input signal passing through a discrete voltage level of the input signal, resulting in a trigger signal that is not responsive to the glitches of interest. It is therefore desirable to have a glitch trigger circuit to provide a trigger signal responsive to glitch signals that are superimposed on sinusoidal power line signals, regardless of whether the glitches are bipolar, positive, or negative polarities and whether the glitch voltage level is less than the power line voltage level. US-A-4 859 872 discloses a synchronizing signal processing circuit in which an input synchonizing signal is delivered to two comparators via a blocking capacitor, the two comparators acting as a window comparator to determine whether the DC-adjusted input signal from the coupling capacitor rises above or falls below respective thresholds. In either case, an OR-gate coupled to the outputs of the comparators produces an output synchronizing signal. However, the circuit cannot deal with negative glitches, and there is no indication as to how to deal with the problem of the signal being superimposed on power line signals. ELECTRONICS INT., vol. 54, no. 6, March 1981, NEW YORK, US, pages 162-163, DAVID M. SMITH "Latch grabs glitches for waveform recorder" discloses a sample and hold circuit which is arranged to determine the voltage of a glitch in a waveform recorder. This is achieved by passing the input waveform through an RC differentiator to a pair of comparators which produce respective outputs in response to significant positive or negative excursions of the input signal. The outputs are both sent to respective inputs of a NOR gate forming part of a latch, so that the latch is triggered in response to a significant positive or negative excursion, this isolating a capacitor from the input voltage and thus allowing the glitch voltage to be maintained by the capacitor. US-A-5 097 147 discloses a trigger circuit for an oscilloscope which applies an input signal to a pair of comparators which are coupled to a logic circuit, the arrangement being such that a trigger signal is produced if the input signal crosses and re-crosses a low threshold as determined by a first comparator without crossing a second, higher threshold as determined by the second comparator. This enables triggering in response to small-amplitude pulses. Summary of the Invention Aspects of the present invention are set out in the accompanying claims. In the preferred embodiment, a 16 kilohertz high pass filter, which blocks the low frequency power line frequency signal while passing higher frequency glitch signals which are typically in the 100 kilohertz to 1 megahertz range, is disposed between the input signal and the inputs of two comparators. The two comparators, one inverting and the other noninverting, provide a trigger signal responsive to the filtered input signal with a voltage level that exceeds either of two set trigger voltage limits where one voltage limit is a positive voltage level and other voltage limit is a negative voltage level. A positive reference voltage and a negative voltage reference provide the desired positive and negative voltage trigger limits to the respective inputs of the two comparators. The output trigger signals from the two comparators are combined using an OR-gate to form the glitch trigger output. The embodiment provides dual-level triggering responsive to positive, negative, and bipolar voltage polarity glitches superimposed on low frequency (50 Hz to 60 hertz) power line signals regardless of the glitch voltage level relative to the power line signal level. The embodiment also enables independent control of both the positive voltage trigger level and the negative trigger level. An arrangement embodying the present invention will now be described by way of example with reference to the accompanying drawings. Brief Description of the Drawings FIG. 1 is a waveform diagram that illustrates the operation of the glitch trigger circuit in accordance with the present invention; and FIG. 2 is a detailed circuit schematic of the preferred embodiment of the present invention. Detailed Description of the Invention Before discussing detailed circuit operation, an overview of the operation of the glitch trigger circuit in accordance with the present invention will be provided. Referring now to FIG. 1, a power line signal forms the sinusoidal input signal, labeled Input Signal. Superimposed on the power line signal are three possible polarities of glitch signals, labeled Bipolar Glitch, Positive Glitch, and Negative Glitch. Glitches, which are transient voltage waveforms of relatively short duration (1 to 10 microseconds typically), may be positive voltage pulses (Positive Glitch), negative voltage pulses (Negative Glitch), or a combination of positive and a negative voltage pulses (Bipolar Glitch). The Input Signal waveform illustrates that glitches can arrive superimposed on the power line signal with unknown voltage polarity and amplitude, and can occur on any phase of the power line signal. The high pass filter rejects the power line signal but passes the glitch signals as illustrated with the Filtered Signal waveform. Superimposed on the Filtered Signal waveform are an upper voltage trigger level labeled Vu and a lower voltage trigger level VL A trigger pulse is generated for the duration of time that the Filtered Signal waveform exceeds the upper voltage trigger level labeled Vu or is less than the lower voltage trigger level VI. The outputs of the comparators are combined to form the output signal labeled Glitch Trigger Output. Referring now to FIG. 2, there is shown the commercial embodiment of the present invention. The input signal is applied to an input terminal 10 labeled Signal Input. The input terminal 10 is connected to the top of a series combination of a capacitor 12 and resistor 14 disposed in series to ground forming a high pass filter with the signal labeled Filtered Signal available at the center tap. In the commercial embodiment, capacitor 12 has a value of 1000 picofarads and resistor 14 has a value of 10,000 Ohms to form a single pole high pass filter with a 16 kilohertz cutoff frequency which rejects low frequency power line signals at input terminal 10 while passing higher frequency glitch signals to form the Filtered Signal. The Filtered Signal is coupled to the non inverting input of a comparator 16 and to the inverting input of a comparator 18. The Positive Reference Voltage is a d.c. (direct current) voltage source coupled to terminal 20 to provide the desired upper voltage limit Vu at the center tap which is coupled to the inverting input of the comparator 16. The Negative Reference Voltage is a d.c. voltage source coupled to terminal 22 to provide the desired lower voltage limit VI at the center tap which is coupled to the non inverting input of the comparator 18. In the commercial embodiment, the polarity of the upper voltage limit Vu is positive with respect to ground and the polarity of the lower voltage limit VI is negative with respect to ground. Comparator 16 provides a signal when the Filtered Input signal voltage is above the upper voltage limit Vu. Comparator 18 provides a signal when the Filtered Input signal voltage is below the lower voltage limit VI. The output of comparator 16 is coupled to an input of an OR-gate 24 and the output of comparator 18 is coupled to another input of the OR-gate 24. The OR-gate 24 is coupled to an output terminal 26 labeled Glitch Trigger Output and provides an output responsive to the output of Comparator 16 and of Comparator 18 to form the output signal. It will be obvious to those having ordinary skill in the art that many changes may be made in the details of the above described preferred embodiments of the invention without departing from the scope of the invention as set out in the accompanying claims. For example, different filters may be substituted to reject unwanted signals of any frequency distinguishable from the frequency of the desired signal to trigger on. Furthermore, the upper trigger voltage limit and the lower trigger voltage limit can be of the same voltage polarity to provide dual-level triggering responsive to signals that do not have a zero crossing.

Claims: A glitch trigger circuit for producing at an output trigger signals in response to glitches on a monitored signal, the circuit comprising a high pass filter (12,14) for receiving the monitored signal and means (16,18,24) for generating trigger signals in response to excursions of the filter (12,14) output which exceed a first predetermined threshold (Vu) which is positive with respect to ground and which fall below a second predetermined threshold (Vl), which is negative with respect to ground, whereby a narrow trigger signal is generated at the output in response to each excursion of a positive, negative or bipolar glitch. A glitch trigger circuit in accordance with claim 1, wherein said filter (12,14) blocks low frequency power line signals and passes high frequency glitch signals. A glitch trigger circuit in accordance with claim 2, wherein said filter (12,14) has a cut off frequency of 16,000 hertz to provide rejection of power line signals while passing glitch signals. A glitch trigger circuit in accordance with any preceding claim, wherein the generating means (16,18,24) comprises: (a) a first comparator (16) having a first input coupled to a first voltage source (70) and a second input coupled to said filter (12,14) to produce an upper limit signal at an output when the voltage of said filtered signal is above said upper limit voltage (Vu); (b) a second comparator (18) having a first input coupled to a second voltage source (22) and a second input coupled to said filter (12,14) to produce a lower limit signal at an output when the voltage of said filtered signal is below said lower limit voltage (Vl) ; and (c) a logical OR circuit (24) having a first input coupled to said first comparator output and a second input coupled to said second comparator output, said logical OR circuit being responsive to said upper limit signal and said lower limit signal to provide a trigger signal.

Inventors: PARLE JONATHAN J, SKELE MARTINS, PARLE, JONATHAN J., SKELE, MARTINS
Applicants: FLUKE CORP, FLUKE CORPORATION
Application year: 1994
Main classification: G01R13
IPC codes: G01R29, G01R13, H03K5